1. Field of the Invention
The present invention relates to X-ray and neutron scattering experimentation devices, and more specifically, the present invention relates to an in-situ test chamber for an electrochemical cell.
2. Background of the Invention
X-ray and neutron scattering and spectroscopy techniques are common in many fields of science, especially those where the characterization of a micro- or nano-structured material is required.
There are a wide variety of X-ray scattering and spectroscopy techniques available to glean important information about a sample's structure. Spectroscopies and scattering methods are favored because they allow nondestructive testing of samples and inexpensive data collection compared to tunneling electron microscopes, scanning electron microscopes, or atomic probing. One of the greatest advantages of these methods is the ability to collect in-situ measurements, that is, measurements performed on the sample during actual use. Many other forms of scattering, spectroscopy and imaging require involved sample preparation or a high vacuum environment, which create unrealistic operating conditions for the sample.
X-ray scattering and spectroscopy involves bombarding samples with X-rays from an X-ray source such as a synchrotron. In some methodologies, a monochromator selects a particular X-ray wavelength, with other optical components focusing the beam before it hits the sample.
X-rays interact with electrons in a sample such that a variety of element specific and structural investigations can be performed. Structural investigations are of particular importance. By measuring the scattering angles of the secondary X-rays, crystallographic insights such as structure and other detail can be obtained. The scattering of X-rays is described by Bragg's law as follows:nλ=2d sin θwhere n is an integer corresponding to the number of X-ray wavelengths that fit between the lattice spacing, λ is the wavelength of the incident X-ray, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering plane. Because the wavelength of the incident X-ray is known and the scattering angle is detected, the lattice spacing can be found from the areas with a high concentration of reflections called Bragg peaks.
Several different X-ray characterization techniques are available, and each provides its own unique information. These techniques have been applied in the field of battery technology, but with limited results. As separate experimentation is required for each spectroscopic method, consistency of the sample is necessary to separate sample data from the background noise. This has proven to be the limiting step, and the problem is compounded by the variability introduced through the use of successive samples instead of a single one.
This is problematic as battery technology has become increasingly important in recent years, with the emphasis on green technologies, electric vehicles, and reducing dependence on foreign oil. For example, a need exists for batteries that can offer sustained voltages over their electrochemical cycle without experiencing tapering voltage as the cycle draws near completion and for batteries that are capable of long-term repeated cycling. Many of the most promising electrode/electrolyte combinations rely on intercalation of ions into the lattice structure of a larger crystal. In order for these reactions to take place, uniform nanostructured materials are needed. A means for characterizing nanostructured materials is currently lacking
Battery technology can profit greatly from accurate nanostructure materials characterization. In order to get the maximum efficiency from electrode materials, the nanostructure changes in the electrode during the electrochemical cycle must be known. But, state of the art sample holders only allow battery components to be investigated over a single or, rarely, a second electrochemical cycle.
The most accurate way to determine the potential value of electrode materials is through in-situ experimentation, that is, during the electrochemical cycle of the battery. In-situ testing not only informs which materials have potential as electrodes but more importantly how they are able to operate effectively. Knowing why some materials work better than others can lead to a more directed and focused scientific inquiry. State-of-the-art ex-situ analysis fails to address this need. Typical ex-situ characterization techniques have several disadvantages: (1) they are extremely labor intensive because each cell requires disassembly to recover the electrode materials and each disassembly runs the risk of “short circuiting” the cell; (2) they require different samples to be studied at different points in the electrochemical cycle, so direct comparison of data is difficult; (3) in practice, ex situ techniques only allow a limited number of points in the cycle to be studied; and (4) they inhibit the investigation of highly-reactive, short-lived intermediaries because the delay involved in recovering the samples during disassembly of the testing device allows such intermediaries to decompose or react before the sample can be characterized.
Separately, currently available in-situ X-ray devices fail to perform the variety of measurements capable in the present invention.
State-of-the-art designs use flexible thin film windows, such as polyimide, aluminized Mylar, and “coffee bags,” to transmit X-rays, which lead to inconsistent battery stack pressure. The flexibility of these materials creates a bulge over the center of the battery, causing a disparity in reaction rates between the center and outer regions of the battery where the stack pressure is maintained. Since the X-ray beam has a fixed height and width of a few hundred micrometers, the largest electrochemical deficiencies and least representative part of the sample coincides with the center of the window in the volume provided by the X-ray beam. Therefore, the area of the battery probed by the X-ray beam produces misleading and unrepresentative information about the reaction and structural changes.
Further, state of the art thin film windows can be permeable to ambient oxygen, nitrogen, and water, and these moieties react with many of the most promising battery components. Thus, the battery components deteriorate faster, limiting the number of possible cycles.
Conversely, rigid windows integrated with existing sample holder technologies allow for only limited data collection. Such windows comprise beryllium substrates and electrode laminates deposited on foil current collectors. These are only compatible with a limited range of X-ray methodologies. The polycrystalline structure of these materials and the problematic elemental impurities create misleading scattering contributions, for example for pair distribution function (PDF) discussed infra.
A need exists in the art for a device and method to characterize battery electrodes in situ. The device and method should enable analysis via X-rays, including PDF characterization, and further facilitate electrochemical analysis. The device and method should also accommodate testing of full size batteries, a plurality of batteries, and battery stacks, all in real time.